The present invention relates to a method of sensitizing lead salt infrared detectors and especially to a method for sensitizing a lead sulfide or lead selenide infrared detector.
In the past, many materials have been known which can be excited by the absorption of infrared energy to provide a useful electrical signal. Some of these detectors of infrared energy take advantage of the fact that a change in conductivity occurs in the material when heating with infrared radiation. This change in conductivity is thus used to modulate a signal impressed across the infrared sensitive material. These device are satisfactory for some purposes but the reaction time in such devices is inherently long since the operation depends entirely upon a thermal affect. Another type of detector employs certain materials which operate on the photoconductive principal such that when the material is excited by absorption of infrared energy, there is a change in conductivity in the material. Certain of these photoconductive materials can be prepared which have very short time constants. Lead salts provide some of the most sensitive materials for the detection of infrared energy at certain wavelengths and especially lead selenide and lead sulfide materials. An infrared detector utilizing these materials usually comprises a thin film of lead selenide or lead sulfide on a substrate with electrical leads connected to opposite sides of the thin film or layer. The thin film of lead salt can be formed by either vacuum evaporation or deposition onto the substrate or by chemically depositing onto the substrate from a solution. The vapor deposition technique has been found to be extremely difficult to control. On the other hand, a chemical deposition from solution technique has also been far from satisfactory in that frequently the precipitation of the lead salt has had poor adherence to the substrate.
Lead salts are typically deposited on the substrates in a manner to form a plurality of infrared detectors simultaneously. First a photomask is made up of the exact shape of the detectors to be formed on the substrate. The photomask is applied directly to the substrate such that the lead salt material is deposited in the exact positions through the photomask to form the desired detectors. Once the lead salt is deposited on the substrate, the lead salt is sensitized by oxidizing the lead salt by positioning the substrate and the lead salt at an elevated temperature for a predetermined time. It is known in the industry that the sensitization of the lead salt material, such as a lead sulfide or lead selenide, is one of the more critical or more difficult processes to control. The sensitization process is the conversion or oxidation of the lead sulfide material using elevated temperatures and time. The resistance of the material is the test most commonly used to determine the useful range of the material as a detector. Presently, the method used is to bake a group of detectors for a set period of time. The resistance of the detectors is measured to determine how long in time the remaining detectors will be baked. The problem associated with this method is the changing conditions in the oven and the room that the oven is residing in which usually gives different results from the evaluation. An alternative method is to bake all the detectors at the same time which has been unacceptable as the resistance target moves from deposition to deposition. Any difference in the deposition process or environmental conditions and the final resistance values is thereby different.
The present invention relates to a process of improving sensitization of the lead salt materials placed on a substrate, such as a quartz plate, having each electrical lead contacting an electrical lead of another detector so that a monitor can be placed across the series connected plurality of detectors formed on a single substrate to monitor the resistance through all of the detectors simultaneously and then immersing the substrate and detectors still connected to the monitor in a heated inert liquid, such as a heated fluorocarbon liquid, at a predetermined temperature until the monitor provides a reading through all of the detectors which can be averaged to determine the desired resistance for each detector. The substrate with the sensitized detectors can be removed from the heated liquid and further processed and separated into individual detectors.
A great variety of prior art patents have dealt with the making of infrared detectors and especially with infrared detectors made with lead salt materials, including both the detectors and processes for making the detectors. In the U.S. Pat. No. 2,997,407, to McLean, a method of production of a lead selenide photodetector is provided which seeks a high degree of reliability in the production of the detector cells which applies a layer of selenide to a suitable substrate, oxidizes the layer of selenide at an elevated temperature, and applies a controlled amount of selenium to the oxidized layer of selenide. In the U.S. Pat. No. 3,178,312, to Johnson, solutions and methods for depositing lead selenide in situ on a substrate of a polycrystaline layer of a thin film of lead selenide by chemical deposition is illustrated. In the U.S. Pat. No. 2,809,132, to Bloem, a method of coating a support with a lead sulfide layer, such as lead sulfide is shown. The Autrey U.S. Pat. No. 3,356,500, shows the production of infrared detector patterns using lead selenide or lead sulfide and delineates the film areas for complex detector array patterns. In the Barrett U.S. Pat. No. 4,689,246, a method of fabricating a lead salt infrared detector array on a silicon wafer which does not employ individual bonded leads between the infrared sensitive elements and the input stages of the multiplexers is provided. In the Barrett patent, U.S. Pat. No. 4,682,032, a Joule-Thomson cryostat having a chemically deposited infrared detector is provided and a method of manufacturing the infrared detector in which the infrared detectors are integrally deposited on a Joule-Thomson cryostat to permit efficient cooling of the detectors. In the Nakamura et al. U.S. Pat. No. 5,138,145, a method for the production of image sensors has a simplified chip mounting technique.
The present invention, on the other hand, deals with an improved technique for sensitizing the lead salt infrared detectors which have been deposited on a substrate and includes making a photomask which interconnects the sensor material leads on the substrate plate for simultaneously monitoring the resistance of an entire batch of sensors being oxidized at an elevated temperature and monitoring the batch during sensitization to determine temperature/time for each batch.